It would be useful for security manager implementations to be able to
alter the client ID of MQTT connections.
This commit supports this functionality by moving the code which handles
the client ID *ahead* of the authentication code. There it sets the
client ID on the connection and thereafter any component (e.g. security
managers) which needs to inspect or modify it can do so on the
connection.
This commit also refactors the MQTT connection class to extend the
abstract connection class. This greatly simplifies the MQTT connection
class and will make it easier to maintain in the future.
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